feat(db): Add make_statement_timeout_budget

[kcons]

Query timeouts are valuable for keeping resource consumption predictable, but for looping or condition code it is hard to apply them effectively. Statement timeouts are per statement, so "let these code below run for up to 10s" is hard to represent with them.
This adds make_statement_timeout_budget, which makes it possible to do:

db_timeout = make_statement_timeout_budget(timedelta(seconds=10))

for item in items:
    with db_timeout():
        perform_query(item)

and know that the cumulative query time isn't going to significantly exceed 10 seconds.
It does introduce the costs of transactions and additional statements, but in the context of behavior that may take seconds, that's usually not a major concern.